The so called environment and energy problem have become widely recognized due to actual evidence of global warming resulting from carbon dioxide emissions, due to peak oil theory and due to energy security problem caused by the growth of developing and middle developed countries.
In order to realize a sustainable society, mass spreading of photovoltaic generation, wind power generation and other renewable electric power generation has gained trends throughout the world. However, increase of renewable electric supply systems, in particular, the widespread introduction of PV systems causes “surplus electricity issue”, which begins to be recognized as an emerging problem, and measures against this problem are needed.
The surplus electricity issue is the problem in which many consumers who has a PV system cannot make full use of the generated power from the PV system sell surplus electricity to the electric power company, resulting in an excessive power supply in the total electric power system. In the prior art, if surplus electricity from electric power generation of a PV system is generated, the amount of electric power that is generated is controlled by controllable power generation facilities such as thermal power plants and the like to thereby optimize the electric power supply in the electric power system and balance the power supply with the amount of electric power that is needed. However, if, in the future, PV systems spread on a large scale, the amount of surplus electricity is expected to exceed the regulating capacity of the thermal power plants etc., and it is recognized that at worst, an imbalance between power supply and demand will lead to power failure, causing a serious problem.
Various kinds of methods have been conventionally investigated, as measures to resolve this surplus electricity issue. For example, these are some known methods: (1) a method to prevent the PV system from generating electric power at a specific date and time (e.g., 11 a.m. to 3 p.m. in the day of the early-May holiday season in Japan) during which of power consumption is low, based on the calendar; (2) a method to prevent multiple PV systems from generating electric power at the same time by sending a control signal using FM broadcasting; (3) a scheduling method that prevents a PV system, in advance, from generating electric power in the power conditioner (PCS) equipped in the PV system; and the like.
However, all of these methods are to avoid the occurrence of surplus electricity by preventing the amount of electric power generated by the PV system, aiming at a technique for operating the PV system at a power level equal to or lower than its power generation capability.
This is a waste of the power generation capability of the PV system, and this is not an efficient method for using a PV system that is to be introduced for the purpose of realizing a sustainable society.
For this reason, another technique has been researched in which electric power is consumed by the user such that the electric power generated by the PV system does not cause reverse power flow at the moment when the PV system initiates control to prevent the generation of electric power.
In general, however, surplus electricity is generated at a time when consumer requirements for electricity are low for facilities equipped with the PV system, and the consumer may be forced to use electricity that he/she does not need and there is concern that this inconvenience may alienate consumers.
Further, patent document 1, for example, proposes a control method of charging and discharging for efficiently storing surplus electricity to the storage battery of the consumer. However, the storage battery is still expensive at present, and it is conceivable that spread of a sufficient number of storage batteries or storage batteries having high enough capacity to solve the above surplus electricity issue will take a considerable time.
Though it is not a technique to present a solution to the surplus electricity problem, patent document 2 discloses an electric power system having a low-voltage system to which the consumers that are equipped with the distributed generator is connected, and a high-voltage system to which the low-voltage system is connected. Patent document 2 discloses that the low-voltage system is separated from the high-voltage system when a problem occurs in the high-voltage system, and that the electric power generated by the distributed generators is shared between the consumers in the low-voltage system.
Of the above-described electric power systems in the background art, the technique that avoids generation of surplus electricity by preventing electric power generation in the PV system is the same as eliminating green electric power generated by the PV system, and this is not an efficient method for using a PV system that is to be introduced for the purpose of realizing a sustainable society.
Further, the technique in which electric power is consumed by the user at the moment when the PV system initiates control to prevent the generation of electric power, the consumer may be forced to use electricity that he/she does not need and there is concern that this inconvenience may alienate consumers.
Moreover, the technique for storing surplus electricity in a storage battery as described in patent document 1 requires consumers to have both a PV system and a storage battery, so that the technique is costly for the consumers.